This invention relates to optical taps and, more particularly, to compensating polarization dependence of the optical taps.
Optical taps are used to sample light from a fiber or beam, and can be based on fused couplers, blazed fiber Bragg gratings, waveguides, beam-splitters, and the like. Typically, these optical taps are polarization dependent. Since the state of polarization in a transmission fiber is unknown and can vary in time, this polarization dependence causes a power uncertainty of the sampled light beam.
An optical channel monitor (OCM) is a good example. The OCM samples optical signals from single mode fibers by diffracting light out of the fiber core into radiation modes via a blazed fiber Bragg grating. This diffraction process is stronger for s-polarized light than for p-polarized light. FIG. 1 shows a polarized beam 102 reflecting from surface 107 of a mirror 101. The plane of incidence 103 is defined as the plane that contains the incident and reflected beams 102 and 104, respectively, and is parallel to the surface of the paper. Plane of incidence 103 also contains normal 106 to surface 107 of mirror 101. The double headed arrows 105 depict p-polarized light where the electric field vector oscillates parallel to, i.e., within, the plane of incidence. The electric field vector of a s-polarized beam oscillates perpendicularly to the plane of incidence. The resulting power inaccuracy can be avoided by scrambling the incoming beam, by employing polarization diversity, or by passive compensation. Scrambling can be realized by varying the state of polarization in the temporal, spatial, or spectral domains. Scrambling is suited for laboratory applications and is typically not cost effective for applications such as channel monitoring. Polarization diversity involves routing the s and p polarization states through different optical paths such that the two states have equal insertion loss. This approach requires a complicated optical path that rarely fits in compact packages. Passive compensation utilizes an optical element that introduces polarization dependent loss (PDL) to undo the PDL of the tap.
The optical channel monitor uses passive compensation, wherein polarization induced power inaccuracy is avoided by reflecting the diffracted free space beam from a mirror. This mirror is positioned such that s-polarization at the grating becomes p-polarization at the mirror. To work effectively, the reflectance of this mirror must compensate grating PDL as a function of wavelength. This spectral dependence can be generated with complex dielectric thin film stacks. FIG. 2A plots the reflectance of the s and p polarization states near the high energy edge of a typical bandpass filter, FIG. 2B shows the ratio of the two curves of FIG. 2A. The ratio in FIG. B can is used to compensate polarization in the C-band. Since the sharp spectral transition is subject to manufacturing variations, the compensation is often imprecise.
These and other problems and limitations of the prior arrangements for attempting to compensate polarization dependence of optical taps are overcome by employing a unique reflective surface.
More specifically, applicant""s invention is a reflective surface that compensates optical tap induced polarization by employing the intrinsic properties of metals rather than complex dielectric thin film stacks.
In one embodiment of the invention, a reflective surface is employed that is a prescribed metal film.
In another embodiment of the invention, a metal surface, polished or otherwise, is employed.
In one example, the metal is preferably tungsten because it exhibits a large difference in the spread of reflectance between the s and p polarization states.
In general, the spectral dependence and magnitude of the ratio of the reflectance (R) of the two polarization states s and p, namely, Rs/Rp, can be accurately matched by choosing amongst four degrees of freedom: selecting the correct metal; adjusting the angle at which the mirror or metal surface reflects a polarized light beam; adding a dielectric layer on top of the metal film; and/or using multiple mirrors or metal surfaces.